Process for regulating the quantity of cold delivered by a refrigerating installation

ABSTRACT

There is provided a process and an installation, operating with an incorporated cascade cycle, in which there is at least one recycling flow drawn off from the cycle mixture at a high pressure the rate of flow of which is regulated, the arrangement being such that at least a part of the recycled flow is (i) expanded to a lower pressure, (ii) combined with a refrigerating stream undergoing evaporation, and (iii) recompressed together with the cycle mixture. The invention is particularly applicable to the liquefaction of natural gas.

United States Patent Bourguet et a1.

1451 Aug. 12, 1975 PROCESS FOR REGULATING THE QUANTITY OF COLD DELIVERED BY A REFRIGERATING INSTALLATION Inventors: Jean Bourguet, Les Vesinet; Joseph Gauberthier, Paris, both of France Teal, Societe des Procedes LAir Liquide et Technip de Liquefaction des Gaz Naturels, Rueil-Malmaison, France Filed: Sept. 20, 1973 Appl. No.: 399,279

Assignee:

Foreign Application Priority Data Sept. 22, 1972 France 72.33672 US. Cl. 62/117; 62/40; 62/197; 62/335 Int. CI. F25B 5/00 Field of Search 62/9, 11, 40, 29, 37, 197, 62/198, 335, 115, 23

References Cited UNITED STATES PATENTS 5/1951 Gilmore 62/117 10/1965 Eld et a1... 62/197 10/1965 Kniel t 62/40 1/1967 Stuart et a1. 62/335 10/1967 Harmens 62/40 3,364,685 1/1968 Perret 62/23 3,593,535 7/1971 Gaurner et a1. 62/23 3,645,106 2/1972 Gaumer et a1. 62/9 3,742,721 7/1973 Bourguet et a1. 62/37 3,747,360 7/1973 Perret 62/40 3,808,826 5/1974 Harper et al... 62/40 3,817,046 6/1974 Aoki et a1. 62/335 FOREIGN PATENTS OR APPLICATIONS 1,557,019 10/1967 France 62/33 895,094 5/1962 United Kingdom. 62/40 1,516,728 3/1965 France 62/40 1,302,989 7/1962 France 62/40 Primary Examiner-Norman Yudkoff Assistant Examiner-Frank Sever Attorney, Agent, or FirmFlynn & Frishauf 5 7] ABSTRACT There is provided a process and an installation, operating with an incorporated cascade cycle, in which there is at least one recycling flow drawn off from the cycle mixture at a high pressure the rate of flow of which is regulated, the arrangement being such that at least a part of the recycled flow is (i) expanded to a lower pressure, (ii) combined with a refrigerating stream undergoing evaporation, and (iii) recompressed together with the cycle mixture, The invention is particularly applicable to the liquefaction of natural gas,

16 Claims, 2 Drawing Figures PATENTEB AUG 1 21915 SHEET PROCESS FOR REGULATING THE QUANTITY OF COLD DELIVERED BY A REFRIGERATING INSTALLATION BACKGROUND OF THE INVENTION This invention relates to refrigeration processes and. more particularly but not exclusively, is concerned with a process for regulating the quantity of cold delivered by a refrigeration installation which utilises the refrigerating cycle known as the incorporated cascade cycle. The invention is particularly concerned with a refrigeration installation which makes it possible to cool, liquefy and possibly super-cool a gas, more particularly but not exclusively a natural gas.

A refrigeration installation operating the incorporated cascade cycle comprises, in general:

i. a compressor, comprising a least one compression stage permitting the compression of at least one cycle mixture in gaseous form from a relatively low pressure to a relatively high pressure;

ii. a condenser having an inlet which communicates with the output of the compressor, and which comprises means for circulating an external refrigerant, permitting at least the compressed cycle mixture to be partially condensed by heat exchange with the external refrigerant to give a first condensed fraction;

iii. a plurality of separators arranged in series, each comprising an inlet, a liquid outlet and a gas outlet, the inlet of the first separator communicating with the outlet of the condenser so that the first con densed fraction obtained at the time of the initial partial condensation can be separated from the gaseous remainder of the cycle mixture (the gaseous remainder of the cycle mixture is known as the residual cycle mixture);

iv. a heat exchange assembly comprising (a) a plurality of condensation pipes arranged in series the inlet of each of which communicates with the gas outlet of one separator and the outlet of each of which communicates with the inlet of the separator next in series, and (b) a vaporisation passages in heat exchange relationship with the condensation pipes, whereby in co-operation with the plurality of separators, fractional condensation of at least the gaseous remainder of the cycle mixture which has been separated from the first condensed fraction is effected at said high pressure while the remainder of the cycle mixture is circulating in said condensation pipes in heat exchange with, and countercurrently with respect to, a refrigerating stream constituting part of the cycle mixture which is passing through said vaporisation passage and is undergoing vaporisation at a vaporisation pressure at least equal to said low pressure, the plurality of separators permitting the collection and separation of further condensed fractions resulting from this fractional condensation;

v. a plurality of expansion devices, of which the upstream side of each device communicates with the liquid outlet of one separator and of which the downstream side of each device communicates with said vaporisation passage, whereby there is effected (a) the expansion from said high pressure to said vaporisation pressure of at least a part of each further condensed fraction, and (b) the combination of the expanded parts with said refrigerating stream; and

vi. a return pipe, of which the upstream end communicates with said vaporisation passage and of which the downstream end communicates with a gas intake of the compressor which is at said vaporisation pressure, whereby there is effected the recompression to said high pressure of at least the vaporised refrigerating stream which constitutes part of the cycle mixture.

The external refrigerant ensuring the initial partial condensation of at least the compressed cycle mixture can either be water (for example, sea water) undergoing reheating, or a refrigerant (for example, propane) undergoing vaporisation, which has been previously compressed, condensed and expanded to a vaporisation pressure in a separate refrigerating cycle.

The refrigerating installation as previously described can use a refrigerating cycle of the open or closed type. In the case of a closed cycle for liquefying, for example, a natural gas, the natural gas circulates in a condensation passage disposed within the heat exchange assembly in heat exchange relationship with the vaporisation passage, but separate from the condensation pipes through which the cycle mixture flows. The refrigerating stream of the cycle mixture is thus vaporised by heat exchange while flowing counter currently with respect to (i) the natural gas which itself is undergoing condensation, and (ii) the cycle mixture which is undergoing fractional condensation. [n the case of an open cycle, again for liquefying natural gas, the natural gas is compressed together with the cycle mixture or combined therewith under high pressure, and thereafter travels through the condensation passages of the heat exchange assembly in admixture with the cycle mixture. The refrigerating stream of the cycle mixture is thus always vaporised in heat exchange with, and while flowing countercurrently with respect to, the cycle mixture and the natural gas. At the end of the fractional condensation, the condensed natural gas is separated from the cycle mixture. Such refrigerating cycles have formed the subject of French Pat. No. 1,302,989, its first certificate of addition No. 80,294, and its further certificate of addition No. 86,485.

According to French Pat. No. 1,516,728, such a cycle operates at two separate vaporisation pressures, the first vaporisation pressure being that at which the cycle mixture is being vaporised by heat exchange with the cycle mixture undergoing fractional condensation, and the second vaporisation pressure being that at which the cycle mixture is being vaporised by heat exchange with a fluid from which it is desired to extract heat, such as a natural gas which is to be liquefied. This cycle will hereinafter be called a two-pressure cycle.

The following features are present in a refrigeration installation according to French Pat. 1,516,728:

a. the compressor comprises a first compression stage and a second compression stage, connected to one another by a connecting pipe under a pressure intermediate the low pressure and the high pressure, this making it possible for the cycle mixture to be compressed in two compression stages, separated by the intermediate pressure; and

b. the return pipe communicates with the inlet of the second compression stage, and this makes it possible to effect the fractional condensation of the residual cycle mixture by heat exchange with the refrigerating stream under a vaporisation pressure substantially equal to the intermediate pressure. In addition, such a refrigeration installation comprises:

a. a second heat exchange assembly comprising a condensation passage in heat exchange relation with a vaporisation passage (hereinafter termed the second vaporisation passage), whereby the extraction of heat from an external fuid, or the condensation of, for example, a natural gas, is effected by heat exchange of the external fluid or of the natural gas, which is passing through said condensation passage, with a second refrigerating stream constituting part of the cycle mixture which stream is flowing through said vaporisation passage and is undergoing vaporisation at a vaporisation pressure substantially equal to the low pressure;

b. a further group of expansion devices, the upstream side of each of which communicates with the liquid outlet of a separator and the downstream side of each of which communicates with the second vaporisation passage, whereby a further part of each further condensed fraction (obtained at the time of the fractional condensation) (a) is expanded to the low pressure and (b) is combined with said second refrigerating stream; and

c. another return pipe, of which the upstream end communicates with the second vaporisation passage and of which the downstream end communicates with the intake of the compressor, whereby said second vaporised refrigerating stream is recompressed from the low pressure to the intermediate pressure, after which it is recompressed together with the first vaporised refrigerating stream, from the intermediate pressure to the high pressure.

In operation, a refrigeration installation of the type just described delivers a quantity of cold which is distributed in accordance with a given temperature gradient. For example, in the case of an installation which utilises a two-pressure cycle, employed for liquefying natural gas, the second refrigerating stream of the cycle mixture, circulating in the second vaporisation passage within the second heat exchange assembly, supplied by its vaporisation and its progressive reheating from a cold temperature up to a hot temperature the quantity of cold which is necessary for cooling, condensing and possibly super-cooling the natural gas from the hot temperature to the cold temperature.

The flexibility required for the operation of a refrigeration installation makes it desirable to be able to regulate the quantity of cold delivered in the working zone of the installation which is between the hot temperature (generally ambient temperature) and the cold temperature (the coldest temperature obtained in the installation). Thus in permanent running conditions, it is frequently necessary to be able to cause a variation, between a maximum quantity and a minimum quantity, in the quantity of cold delivered for the purpose of adapting this to the quantity of heat injected into the installation, (which varies substantially within the same limits). Likewise, under transient running conditions (stopping or starting of the installation), it is imperative to be able to cause a variation in the delivered quantity of cold between a zero initial quantity and a final quantity corresponding to permanent running conditions (or vice versa). For example, in the case of an installation for liquefying natural gas, if the rate of flow of natural gas which is treated is capable of varying from zero up to a maximum value, it is imperative to adapt the delivered quantity of cold to the variable quantity of heat injected by the circulation of the natural gas in the installation.

For a refrigerating installation of the aforementioned type, delivering a large quantity of cold units, especially for liquefying natural gas, the compressor is generally a compressor of the axial or centrifugal type. It it is possible to cause a variation in the rate of flow of the cycle mixture which is drawn in, either by causing a variation in the speed of rotation of the compressor, or by modifying the position of the directional blades of the stator in the case of an axial compressor, the flexibility thus obtained is very limited. Consequently if, for example, the rate of flow of the cycle mixture drawn in by an axial or centrifugal compressor is reduced so as to cause a variation in the quantity of cold supplied by the installation, it very quickly becomes impossible to reduce the quantity drawn in without altering the pumping stage of the compressor. Consideration of such problems has led to the invention defined below.

In order to be able to regulate the quantity of cold delivered by a refrigerating installation of the type described above (to which the invention relates), the installation comprises in addition at least one recycling circuit comprising at least one variable delivery expansion device, of which the upstream side communicates with a gas outlet of the compressor means at a pressure at most equal to the high pressure, and of which the downstream side communicates with a gas inlet of the compressor at a lower pressure at least equal to the low pressure. This recycling circuit is generally arranged around and in the intermediate proximity of the compressor, that is to say, in that part of the installation which is at ambient temperature. It is thus possible to regulate at least one recycling flow which is drawn off from the cycle mixture at the higher pressure before or after it has been cooled, and is when successively expanded, at least in part, to the lower pressure after which the expanded part is recompressed together with the vaporised refrigerating stream. It is thus possible to regulate the quantity of cycle mixture circulating in the cold part of the installation and consequently to regulate the quantity of cold delivered and this is obtained without altering the pumping rate of the compressor.

When the quantity of cold delivered by the refrigeration installation is regulated by causing a variation in the recycling flow around the compressor, at least a part of the power delivered by the compressor is then dissipated into the recycling circuit in the form of heat, vibration, etc.

Even if such energy dissipation into the recycling circuit does not in general raise any particular problem in the case of a refrigeration installation of low refrigerating power, it was discovered that this no longer applies to installations of greater refrigerating power, this being especially the case with liquefying installations of high capacity for natural gases. In other words, it was found that, beyond a certain capacity in a liquefying installation for natural gas, it is imperative to provide a solution for new problems which are caused by the size of the refrigeration installation.

In the first place, an increase in the capacity of a liquefying installation for natural gas means that the power dissipated in the recycling circuit may be extremely large. As an example, an installation which liquefies a nominal flow of 6500 cubic metres per day of liquid natural gas uses of the order of 66 megawatts of power for compression at the normal production rate, and 80 megawatts for maximum production rate. The result of this is that, if a recycling circuit such as that described above is employed, the power dissipated in the recycling circuit can be extremely large. The recycling circuit is consequently subjected to considerable vibration and the metal fatigue which results therefrom can very quickly lead to a breakdown in the equipment. This danger of breakdown is further accentuated by the following new problem. Because of the increased capacity of a large installation for liquefying natural gas, the various fittings of the installation themselves be comes large. This is also the case with the various recy cling pipes of the installation. Hence, the compact nature of the pipelines, which can be characterised by the ratio between their radius and their thickness, which formerly did not exceed 20, may reach 75 for large capacity installations. The result thereof is that the mass of metal per unit volume of the recycling circuit be comes too small to support the considerable effects of the vibrations. This phenomenon thus amplifies the disadvantage referred to above.

Consequently, the increase in the capacity of an installation for liquefying natural gas affects the metal components being used, and particularly the recycling circuit or circuits in a new working zone, in which it is essential to take into account new physical phenomena which have so far never been encountered in this type of installation, namely, aeroelasticity phenomena which are well known in aviation. Consequently, in large capacity installations, the recycling equipment behaves as a thin shell structure and is in general subjected to a destructive fatigue.

It is an object of the present invention to improve the means for regulating the quantity of cold delivered by a refrigeration apparatus of the type set forth in such a way that the increase in nominal refrigerating power of such an installation does not result in the danger of breakdown of the recycling equipment.

SUMMARY OF THE INVENTION According to the present invention, there is provided a process for regulating the quantity of cold delivered by a refrigeration installation operating an incorporated cascade cycle, in which:

a. at least one cycle mixture is compressed in gaseous form from a lowpressure to a high pressure, in at least one compression stage;

b. the compressed cycle mixture is partially condensed, by heat exchange with an external refrigerant, and a first condensed fraction is separated therefrom;

c. fractional condensation of at least the gaseous residual cycle mixture which has been separated from the first condensed fraction is carried out at said high pressure by heat exchange with, and while flowing counter-currently with respect to, a refrigerating stream which constitutes a part of the cycle mixture which stream is undergoing vaporisation at a vaporisation pressure at least equal to said low pressure, whereby a plurality of further condensed fractions is separated from the residual cycle mix- 6 ture; d. at least a part of each of the condensed fractions is expanded from said high pressure to said vaporisation pressure, and the expanded parts thus obtained are united with said refrigerating stream; and

e. at least said vaporised refrigerating stream constituting a part of the cycle mixture is recompressed to said high pressure,

wherein the quantity of cold delivered by the installation is regulated by controlling the rate of flow of at least one recycling flow which is drawn off from the cycle mixture at a pressure at most equal to said high pressure, the arrangement being such that when the recycling flow takes place at least a part of the recycling flow is (i) expanded to a pressure substantially equal to said vaporisation pressure; (ii) combined with said refrigerating stream at a stage in the process not later than when said at least a part of the first condensed fraction, after having been expanded to said vaporisation pressure, is combined with the refrigerating stream; and (iii) recompressed together with the said refrigerating stream when the refrigerating stream has been vaporised.

The invention also provides a refrigerating installation which comprises:

i. a compressor comprising at least one compression stage permitting the compression of a cycle mixture in gaseous form from a relatively low pressure to a relatively high pressure;

ii. a condenser having an inlet which communicates with the output of the compressor, and which comprises means for circulating an external refrigerant;

iii. a plurality of separators arranged in series, each comprising an inlet, a liquid outlet and a gas outlet, the inlet of the first separator communicating with the outlet of the condenser;

iv. a heat exchange assembly comprising (a) a plurality of condensation pipes arranged in series the inlet of each of the which communicates with the gas outlet of one separator and the outlet of each of which communicates with the inlet of the sepa' rator next in series, and (b) a vaporisation passage in heat exchange relationship with the said condensation pipes;

. a plurality of expansion devices, of which the upstream side of each device communicates with the liquid outlet of one separator and of which the downstream side of each device communicates with said vaporisation passage;

vi. a return pipe, of which the upstream end communicates with said vaporisation passage and of which the downstream end communicates with a gas intake of the compressor which is at a pressure at least equal to said low pressure; and

viii. at least one recycling circuit comprising at least one adjustable flow expansion device, of which the upstream side communicates with a gas outlet of the compressor at a high pressure at most equal to said high pressure and of which the downstream side communicates with the gas intake of the compressor at a pressure at least equal to said low pressure, wherein the downstream side of the adjustable flow expansion device of the recycling circuit communicates with said vaporisation passage.

In practising the process of this invention, the recycling flow which has been expanded to the vaporisation pressure is combined with the refrigerating stream, at the latest when the latter has been combined with at least a part of the first condensed fraction after the first condensed fraction has been expanded to the vaporisation pressure. Consequently, in accordance with the invention, the downstream side of the expansion device of the recycling circuit communicates with the vaporisation passage reserved for the refrigerating stream undergoing vaporisation.

The invention provides the following advantages. In the first place, since the junction between that part of the recycling circuit which is at the vaporisation pressure and the circuit of the refrigerating stream (i.e. the vaporisation passage and return pipe) which is also at the vaporisation pressure is moved back as far as possible into the cold part of the refrigerating installation, at least a part of the vaporisation passage and the return pipeare integrated into the recycling circuit. A large part of the total mass of the installation thus participates in the dissipation of the vibrational energy resulting from the recycling. Secondly, since the recycling circuit includes the last part of the vaporisation passage, in which the first condensed fraction is vaporised by heat exchange with the refrigeration stream, the dissipated thermal energy resulting from recycling is absorbed by the cycle mixture and is transferred by the latter, after compression, to the external refrigerant. The dissipated thermal energy, instead of directly reheating the installation, is recovered and methodically evacuated from the installation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described by reference to the accompanying drawings which illustrate a preferred embodiment of the invention, and in which:

FIG. 1 shows diagrammatically an installation for liquefying natural gas, permitting the liquefaction of about 1 /2 thousand million cubic metres of natural gas per year, i.e about 187,000 cubic metres per hour (at n.t.p.) of natural gas, using a two-presence incorporated cascade cycle; and

FIG. 2 represents the variations in the total delivery recycled around the compressor used in the installation shown in FIG. 1, as a function of the throughput of liquefied natural gas.

The refrigeration plant which is shown in FIG. 1 comprises first of all a compression device 1 (an axial compressor), driven by a steam turbine 2, which comprises a first compression stage 3 and a second compression stage 4 connected to one another by a connecting pipe 5 under an intermediate pressure (between the low pressure and the high pressure) at which the output of the stage 3 and the intake of the stage 4 function. A refrigerator unit 6 is disposed on the connecting pipe 5.

A condenser 7 has an inlet communicating with the delivery side of the compression means 1 by means of a pipe 8 on which is disposed a refrigerating unit 9. The condenser 7 comprises circulation means for an external refrigerant, such as water.

A plurality of separators 10, 11 and 12, arranged in series, each comprise an inlet indicated by the appropriate reference number and the index letter a, a liquid outlet indicated by the appropriate reference number and the index letter b and a gas outlet indicated by the appropriate reference number and the index letter c. The inlet a of the first separator 10 communicates with the outlet of the condenser 7.

A first heat exchange assembly, represented at 13, comprises a first exchanger 14 and a'second exchanger 15. This heat exchange assembly 13 comprises a plurality of condensation pipes 16, 17 and 18 arranged in series. The first pipe 16 is disposed in the exchanger 14, while the pipes 17 and 18 are disposed in the exchanger 15. The inlet end of each condensation pipe, bearing the appropriate reference number together with the index letter a, communicates with the gas outlet of a separator: the gas outlet with the inlet 16a, the gas outlet 1 1c with the inlet 17a and the gas outlet 12c with the inlet 18a. The outlets of the first two condensation pipes 16 and 17 communicate with the inlets of the following separators: outlet 16]) with inlet 11a, outlet 17b with inlet 12a. The heat exchange assembly 13 further comprises a vaporisation passage comprising the interior (grid side) of the exchanger 13, the connecting pipe 71 and the interior of the exchanger 14 (grid side) in heat-exchange relationship with the condensation pipes 16,17 and 18.

The installation comprises second heat exchange assembly 22, comprising a third exchanger 23 and a fourth exchanger 24. This assembly comprises a condensation passage comprising a pipe 25 arranged within the exchanger 24, a connecting pipe 26 between the exchangers 23 and 24 and a pipe 27 arranged within the exchanger 23. This condensation passage is in heat exchange relationship with a second vaporisation passage which is defined by the interior of the exchanger 23 (grid side), a connecting pipe 28 and the interior of the exchanger 24 (grid side).

The installation also comprises a first group of expansion devices in the form of valves 19, 20 and 21 and a second group of expansion devices in the form of valves 29, 30 and 31. In eachcase the upstream side of an expansion device communicates with the liquid outlet of a separator; thus liquid outlet 10b communicated with the expansion valve 19, liquid outlet 11b with the expansion valves 20 and 29 simultaneously, and liquid outlet 12b with the expansion valve 21 and the expansion valve 30 simultaneously. The upstream side of the expansion valve 31 communicates with the outlet 18b of the condensation pipe 18 only. The downstream side of each of the expansion valves 19, 20 and 21 communicates with the vaporisation passage of the first heat exchange assembly 13. The downstream sides of the expansion valves 29, 30 and 31 communicate with the vaporisation passage of the second heat exchange assembly 22.

The upstream and downstream ends of a return pipe 32 communicate respectively with the vaporisation passage of the first heat exchange assembly 13 (more specifically with the interior of the exchanger 14) and with the inlet of the second compression stage 4 of the compressor which is under the intermediate pressure. The upstream and downstream ends of another return pipe 33 communicate respectively with the vaporisation passage of the second heat exchange assembly 22 (more specifically with the interior of the exchanger 24) and with the intake of the compressor (that is, with the entry side of the first compression stage 3 at the low pressure).

The functioning of one example of a two-passage cycle, using the installation represented in FIG. 1, will now be described. Rates of flow are expressed in metric tons per hour (t/h), pressures in absolute atmospheres (at., l at.= 1.0l3 bars) and temperatures in degrees Centrigade (C). In this cycle, the condensed fraction obtained from the initial, partial condensation of the cycle mixture is called the first condensed fraction, while the first of the further condensed fractions is called the second condensed fraction; subsequent further condensed fractions are given ascending ordinal numerical references so as to facilitate description.

A first part ofa cycle mixture, having an average molecular weight of 29 and comprising methane, nitrogen, ethane, propane, butane and pentane, arrives by way of the return pipe 33 and is compressed in gaseous form in the first compression stage 3 from a low presence of 1.5 at. to an intermediate pressure of 5.5 at. Then this first, compressed part, which has been cooled while at the intermediate pressure in the cooling unit or refrigerator 6, is further compressed in gaseous form in the second compression stage 4 to a high pressure of 38 at. together with a second part of the cycle mixture which has an average molecular weight of 34 and comprises the same constituents as the first part of the cycle mixture. The second part of the cycle mixture arrives at compression stage 4 via return pipe 32.

The compression cycle mixture is thereafter cooled in the cooling unit 9, then partially condensed in the condenser 7, by heat exchange with an external refrigerant. A first condensed fraction is then separated out from the cycle mixture in the first separator 10. The whole of this first condensed fraction, amounting to 340 t/h, is then expanded in the expansion valve 19, and reunited with a refrigerating stream which is circulating inside the exchanger 14 (i.e. in the vaporisation passage of the first heat exchange assembly 13).

The residual gaseous cycle mixture, at the high pressure, which has been separated from the first condensed fraction in the separator 10, is then subjected to fractional condensation in the first heat exchange assembly 13 which communicates with the two separa tors l1 and 12. The fractionation takes place successively in the condensation pipes 16, 17 and 18, by heat exchange with, and counter-currently with respect to, a refrigerating stream circulating in the previously defined vaporisation passage, which stream is undergoing vaporisation at a vaporisation pressure which is substantially equal to the intermediate pressure (5.5 at.). A second condensed fraction and a third condensed fraction are separated at, respectively, the outlet 16]; of the first condensation pipe 16 and the outlet 17b of the second condensation pipe 17; these two condensed fractions are collected in the second separator 11, at a temperature of -22C, and in the third separator 12, under a temperature of 80C, respectively. A fourth condensed fraction, condensed at a temperature of -l30C, is not separated but is directly discharged from the upper part of the condensation pipe 18 located within heat exchanger 15.

A first part (170 t/h) and a second part (272 t/h) of the second condensed fraction which has been collected in the separator 11 are expanded respectively, to the intermediate pressure in the expansion valve 20 and to the low pressure in the expansion valve 29. The first part of the second condensed fraction is reunited at intermediate pressure with the vaporised refrigerating stream circulating in the vaporisation passage of the first heat exchange assembly 13, and the second part of the second condensed fraction is reunited at the low pressure with the second refrigerating stream circulating in the vaporisation passage of the second heat ex change assembly 22. The same operations take place with a first part (135 t/h) and a second part (54 t/h) of the third condensed fraction collected in the separator 12, that is these two parts are expanded in the expansion valves 21 and 30, respectively, to the intermediate pressure and to the low pressure, respectively, and are thereafter introduced into the vaporisation passages of, respectively, the heat exchangers l5 and 23. The whole of the fourth condensed fraction (46 t/h) is expanded in the expansion valve 31 to the low pressure and is introduced into the vaporisation passage of the second heat exchange assembly 22, at the upper part of the exchanger 23.

The introduction of a part of the different condensed fractions into the vaporisation passage of the first heat exchange assembly 13 permits a refrigerating stream to be obtained which is undergoing vaporisation at the intermediate pressure while circulating in the said passage, in a vertical descending direction in the exchanger 15 and then in a vertical ascending direction in the exchanger 14. This stream, formed at the start by the first part of the third condensed fraction, then reunited successively with the first part of the second condensed fraction and with the first condensed fraction, is-vaporised and progressively reheated up to ambient temperature.

Likewise, the introduction of a part of the different condensed fractions into the vaporisation passage of the other heat exchange assembly 22 permits another refrigerating stream to be obtained which is undergoing vaporisation at the low pressure while circulating in the second vaporisation passage in a vertical descending direction in the exchanger 23 and then in a vertical ascending direction in the exchanger 24. This stream, formed at the start by the whole of the fourth condensed fraction, then successively reunited with the second part of the third condensed fraction and with the second part of the second condensed fraction, is vaporised and is progressively reheated up to ambient temperature, by heat exchange in counter-current with natural gas being cooled, condensed and then supercooled from the ambient temperature down to a final cold temperature of about l62C, at a pressure of 42 at., The natural gas enters the installation through a line which joins the condensation pipe 25 within the exchanger 24, and then passes via line 26 to condensation pipe 27 within the exchanger 23. The natural gas has the following composition:

Finally, the second refrigerating stream which has been undergoing vaporisation in the body of the second heat exchange assembly 22 passes via return pipe 33 to the compression stage 3 where it is recompressed from the low pressure to the intermediate pressure and then,

together with the refrigerating stream vaporised in the compression stage 4, is further compressed from the intermediate pressure to the high pressure.

So as to be able to regulate the quantity of cold delivered by the installation as a function of the quantity of heat injected into the installation, that is to say, as a function of the rate of flow of natural gas to be liquefied, the following recycling circuits are provided:

A first recycling circuit 34 comprising an adjustable flow expansion valve 38, of which the upstream side communicates with the gas outlet 12c of the separator 12, and consequently via the separators 10, 11 and 12 and the condensation pipes 16 and 17 with the high pressure gas outlet of the compressor 1, and of which the downstream side communi cates with the vaporisation passage of the assembly 13, more specifically with connecting pipe 71, and consequently via the final part of the vaporisation passage and the return pipe 32 with the intermediate pressure gas inlet of the compressor 1;

A second recycling circuit 35 comprising an adjustable flow expansion valve 39, of which the upstream side communicates with the gas outlet 12c of the separator 12, which is at the high pressure, and of which the downstream side communicates with the vaporisation passage of the second heat exchanger assembly 22, and more specifically with the lower part of the interior of the exchanger 23,

Y which is at the low pressure;

A third recycling circuit 36 comprising an adjustable flow expansion valve 40, of which the upstream side communicates with the pipe 8 between the cooling unit 9 and the condenser 7, which is at the high pressure, and of which the downstream side communicates with the lower part of the exchanger 14, which is at the intermediate pressure; and

A fourth recycling circuit 37 comprising a variable flow expansion valve 41, of which the upstream side communicates with the connecting pipe between the compression stages 3 and 4, which is at the intermediate pressure; and of which the downstream side communicates with the lower part of the exchanger 24, which is at the low pressure.

There is also provided around the compression stage 4 a stop recycling circuit 42, which includes an expansion valve 70, and a stop recycling circuit 43 around the compression stage 3 which includes an expansion valve 44. The pipes for these recycling circuits have a ratio between their radius and their thickness which is at most equal to 70.

Hereinafter there are described different procedures for regulating the quantity of cold delivered by the installation shown in FIG. 1, as a function of the quantity of heat injected into the installation, i.e. as a function of the throughout of natural gas to be liquefied. These different procedures depend upon different combinations of the varying recycling circuits which have been described above.

According to one method of regulation, all four recycling circuits 34, 35, 36 and 37 are brought into'use. In describing this method of regulation, reference will be made to FIG. 2, which shows a plot of the quantity of natural gas to be liquefied (on the abscissae), expressed as a percentage of the nominal production of the installation, against the total recycled throughput (on the ordinates), expressed as a percentage of a reference throughput of the cycle mixture, which is that quantity delivered at the high pressure by the compressor 1 necessary for liquefying l07% of the nominal production of liquefied natural gas. The solid line' on the plot relates to the first method of regulation (which is about to be described) while the dashed line relates to the second method of regulation which will be described later.

ln order to explain the functioning of the regulation, the effects resulting when the production of liquefied natural gas decreases are described below.

On starting, the installation delivers a quantity of cold corresponding to the functioning point R, (see FIG. 2), that is to say, to a production equal to 107% of the nominal production. Under these conditions, the power consumed by the compressor is 73.5 megawatts for a rotational speed of 3680 revolutions per minute. 400 Tons per hour are drawn in by the first compression stage 3 and 1046 tons per hour by the second compression stage 4.

When the rate of flow of natural gas to be liquefied is reduced from the functioning point R to the functioning point R corresponding to 93% of the nominal production, the throughput of the cycle mixture is regulated by reducing the speed of rotation of the compressor 1 and by modifying the orientation of the stator blades of the second compression stage 4. At the functioning point R the power consumed by the compressor is 58.6 megawatts for a speed of rotation of 3200 revolutions per minute. 345 Tons per hour and 900 tons per hour are drawn in respectively by the compression stages 3 and 4.

From the functioning point R when the production of liquefied natural gas decreases,the speed of rotation of the compressor remains constant and equal to 3200 revolutions per minute (low running speed). Starting from R the recycling circuit 35 is brought into operation. Under these conditions, a first recycling flow is taken, at the high pressure and at the gas outlet of the separator 12, from the residual cycle mixture in gaseous form which is undergoing fractional condensation (after the separation of the second and third condensed fractions which are collected in the separators 11 and 12 respectively). This first recycling flow is expanded to the low pressure in the expansion valve 39, is reunited with the second refrigerating stream in the bottom of the exchanger 27 (before said stream is reheated up to ambient temperature), and is finally recompressed to the high pressure in the compressor 1 with the second vaporised refrigerating stream. The combining of the first recycling flow in the circuit 35 with the second refrigerating stream is effected after the latter has been united with a part of the second condensed fraction which has been expanded in the expansion valve 29. Between the functioning points R and R the first recycling flow is regulated from a minimum flow of zero up to a flow of the order of 3% of the reference flow of the cycle mixture (curve in solid line). From R to R the inclination of the stator blades of the second compression stage 4 is modified. At the point R (87% of the nominal production), about 3% of the reference flow of the cycle mixture is recycled through the recycling circuit 35. Under these conditions, the power consumed by the compressor 1 is 55 megawatts.

Starting from the functioning point R the power consumed remains constant and equal to 55 megawatts. The inclination of the stator blades of the second compression stage 4 also remains constant from the point R When the production of liquefied natural gas is decreased from R a second recycling flow is drawn off, at the high pressure and at the gas outlet 12c of the separator 12, frm the residual cycle mixture in gaseous form which is undergoing fractional condensation. This second recycling flow (in the circuit 34) is then expanded in the expansion valve 33 to the intermediate pressure, and is combined with the first refrigerating stream circulating in the pipe 71 (before the latter is united with the first condensed fraction which has been explained to the intermediate pressure in the expansion valve 19, but after a part of the second condensed frac tion whichhas been expanded to the intermediate pressure in the valve 20 has been added to the first refrigerating stream). The. second recycling flow is finally recompressed to the high pressure in the second compression stage 4, together with the two vaporised refrigerating streams. The second recycling flow in the circuit 34, like the first recycling flow is tapped off from the residual cycle mixture, after separation of the secnd and third condensed fractions which have been collected in the separators l1 and 12 respectively. From R to R the second recycling flow, in the circuit 34, is regulated from a minimum value of zero up to a maximum'value. At the functioning point R corresponding to the liquefaction of 50% of the nominal production of liquefied natural gas (during which time the first recycling flow is at its maximum value) about 35% of the reference flow of the cycle mixture is recycled simultaneously by the circuits 34 and 35.

When the production of natural gas decreases from the functioning point R the recycling circuits 36 and 37 are brought into operation simultaneously. A third recycling flow is tapped off into the recycling circuit 36 at high pressure from the compressed cycle mixture circulating in the pipe 8, before the mixture undergoes partial condensation in the condenser 7 but after it has been cooled in the cooling unit 9. The third recycling flow, in the circuit 36, is tapped off at the valve 40 and is then expanded to the intermediate pressure. This expanded recycling flow is combined with the first refrigeration stream, which is circulating in the exchanger 14, at about the same time as the first condensed fraction (which has been expanded to the intermediate pressure in the expansion valve 19) joins said stream. A fourth recycling flow at intermediate pressure is tapped off from the cycle mixture circulating in the pipe between the two compression stages 3 and 4, after the mixture has been cooled in the coolingunit 6, into the recycling circuit 37. The fourth recycling flow as thus drawn off is expanded to the low pressure in the expansion valve 41, and this expanded flow is combined with the second refrigerating stream, before it is reheated to ambient temperature, in the bottom of the exchanger 24. From R to R the third recycling flow (in the circuit 36) and the fourth recycling flow (in the circuit 37) are regulated and increased and simultaneously from an initial value of zero up to a final value. At the functioning point R the production of liquefied natural gas is zero, and the total recycled flow corresponds to about 80% of the reference flow of the cycle mixture which is substantially equal to the rate of flow delivered by the compressor when this latter has a speed of 3200 revolutions per minute.

The functioning zone contained between the positions R and R corresponds to a transient functioning TF of the refrigerating installation (starting and stopping) during which the quantity of cold delivered by the installation is regulated between an initial zero quantity (R and a final or minimum quantity (R In this transitional zone, for reducing the quantity of delivered cold, the third and fourth recycling flows, in the circuits 36 and 37, (which may be called transitional recycling flows) are thus simultaneously increased up to an initial rate of, flow of about 80% 0f the reference flow.

When the production of natural gas is raised from an initial zero production (corresponding to the functioning point R up to maximum production rate corresponding to the functioning point R the regulating sequences which have been described above are carriedout in the reverse order. Thus when the installation is started up, the flow in the recycling circuits 36 and 37 is initially at the maximum value; during the transient functioning of the installation, the recycling flows 36 and 37 are simultaneously decreased to a zero rate of flow.

When the production of liquefied gas increases beyond the levels within the transient functioning zone TF, the installation enters a permanent functioning zone PF during which the quantity of cold delivered is regulated between the minimum quantity (R and the maximum quantity (R During the permanent operation of the installation, i.e. between the functioning points R and R if it is desired to reduce the quantity of cold delivered, the first recycling flow, 35, and then the second recycling flow, in the circuit 34, are successively increased up to a maximum flow corresponding, for the two circuits 34 and 35, to a total recycled flow of 35% of the reference flow as described above. The first and second recycling flows may thus be called the permanent recycling flows since their use is associated with the running of the installation under permanent conditions. Conversely, between the functioning points R and R if it is desired to increase the quantity of cold delivered, the second recycling flow and then the first recycling flow are successively reduced to a minimum flow of zero. The recycling flows in the circuits 35 and 34 respectively are thus regulated to their maximum value during transient running periods of the installation.

According to a second regulating procedure, only the third and fourth recycling circuits 36 and 37 are brought into operation, and the corresponding recycling flows are regulated. When the production of liq ,uefied natural gas increases from the functioning point R (zero delivery of natural gas) to the functioning point R (maximum delivery of natural gas), the recycling flows 36 and 37 are first simultaneously reduced froman initialvalue to a minimum value (zero for the recycling flow in the circuit 36 but not zero for the recycling flow in the circuit 37), between the points R and R as shown in dashed lines in FIG. 2. Then, starting from thepoint R as the production of liquefied natural gas continues to increase, the recycling flowin the circuit 37 is reduced to a zero values (the flow in V the circuit 36 is still zero), and simultaneously the orientation of the stator blades of the second compression stage 4 is modified. When the rate of production of natural gas is decreasing between the points R and R each of these regulating sequences is carried out in the reverse order to that just described. 7

What is claimed is: 1. In a process for regulating the frigorific power delivered by a refrigeration installation operating an in corporated cascade cycle wherein i a. at least one cycle mixture is compressed in gaseous form from a lower pressure to a higher pressure, in at least one compression stage, b. the resulting compressed cycle mixture is partially condensed, by heat exchange with an external retherefrom.

c. progressive condensation of at least the residual gaseous cycle mixture is affected at said higher pressure by heat exchange with, and while flowing counter-currently with respect to, a diphasic refrigerating stream comprised of at least a part of said cycle mixture, which stream is undergoing vaporization at a vaporization pressure at least equal to said lower pressure, whereby at least a last liquid fraction is obtained from the residual cycle mixture,

d. at least a part of each of the liquid fractions of said cycle mixture is expanded from said higher pressure to said vaporization pressure, and the expanded liquids thus obtained are combined into said refrigerating stream, and

e. at least said vaporized refrigerating stream formed in (d) and constituting at least a part of said cycle I mixture is recompressed in gaseous form to said higher pressure.

the improvement which comprises i. withdrawing at least a first gaseous recycle stream .from said cycle mixture in gaseous form, under a pressure at most equal to said higher pressure and above said lower pressure,

ii. expanding to said vaporization pressure at least a gaseous portion of said withdrawn gaseous stream,

iii. combining said gaseous expanded portion with said refrigerating stream, before or when said at least a part of liquid fraction (A), after having been expanded to said vaporization pressure, is combined with said refrigerating stream,

iv. recompressing in gaseous form said combined portion formed in (iii) together with said refrigerating stream, when the latter has been vaporized, and

v. controlling the flow-rate of said expanded gaseous portion of said first recycle stream.

2. A process according to claim 1, wherein said expanded gaseous portion of said first recycle stream is combined with said refrigerating stream when said at least a part of liquid fraction (A), after having been expanded to said vaporization pressure, is combined with said refrigerating stream.

3. A process according to claim 1, wherein said expanded gaseous portion of said first recycle stream is combined with said refrigerating stream before said at least a part of liquid fraction (A), after having been expanded to said vaporization pressure, is combined with said refrigerating stream.

4. A process according to claim 3, wherein by a progressive and fractional condensation according to step (c), there is obtained and separated from the residual cycle mixture at least a second and further liquid fraction, intermediate said first and last liquid fractions, and wherein said expanded gaseous portion of said first recycle stream is combined with said refrigerating stream, after at least a part of said second liquid fraction, which has been previously expanded to said vaporization pressure, is combined with said refrigerating stream.

5. A process according to claim 1, wherein said gaseous cycle mixture is cooled after being compressed according to step (a) and before being subjected to partial condensation according to step (b), and wherein said first gaseous recycle stream is drawn off at said higher pressure from said compressed cycle mixture in gaseous form before partial condensation thereof but after said compressed gaseous cycle mixture has been cooled.

6. A process according to claim 1, wherein said first gaseous recycle stream is drawn off at said high pressure from the residual cycle mixture in gaseous form, during the progressive condensation thereof according to step (c).

7. A process according to claim 6, wherein by a progressive and fractional condensation according to step (0), there are obtained and separated from said residual cycle mixture second and third further liquid fractions, intermediate said first and last liquid fractions, and wherein said gaseous recycle stream is withdrawn from said residual cycle mixture in gaseous form, after the separation from said residual recycle stream of the second and third fractions.

8. A process according to claim 1, wherein said incorporated cascade cycle includes the steps:

f. compressing said cycle mixture is gaseous form according to step (a) in two compression stages separated by a conduit at a pressure intermediate said higher pressure and said lower pressure,

g. progressively and fractionally condensing according to step (c), by heat exchange with a first diphasic refrigerating stream, constituting a first part of said cycle mixture, which first diphasic refrigerating stream is undergoing vaporization under a first vaporization pressure substantially equal to said intermediate pressure, and obtaining and separating from said residual cycle mixture at least a second and further liquid fraction (B), intermediate said first and last liquid fractions,

h. extracting the heat content of an external fluid by heat exchange of said external refrigerant with a second diphasic refrigerating stream, constituting a second and remaining part of said cycle mixture, which second diphasic refrigerating stream is undergoing vaporization under a second vaporization pressure substantially equal to said lower pressure,

j. expanding to said first and second vaporization pressures, respectively, a first part and a second and remaining part of at least said second liquid fraction (B) of said cycle mixture, obtained during the progressive and fractional condensation according to step (g); and combining the expanded first liquid part and the expanded second liquid part thus obtained with said first diphasic refrigerating stream and with said second diphasic refrigerating stream, respectively,

k. recompressing in gaseous form said vaporized second diphasic refrigerating stream from said lower pressure to said intermediate pressure, and then recompressing the resulting recompressed stream in gaseous form from said intermediate pressure to said higher pressure, together with said vaporized first diphasic refrigerating stream,

and wherein the frigorific power delivered by the refrigeration installation is regulated by:

l. withdrawing at least a second gaseous recycle stream from said cycle mixture in gaseous form, under a pressure at most equal to said higher pressure and above said lower pressure,

m. expanding to said second vaporization pressure at least a gaseous portion of said second withdrawn recycle stream,

n. combining said gaseous expanded portion of said second recycle stream with said second diphasic refrigerating stream before the latter is substantially reheated to ambient temperature,

0. recompressing in gaseous form said combined portion of said second recycle stream, together with said second diphasic refrigerating stream, when the latter has been vaporized,

p. controlling the flow-rate of said gaseous expanded portion of said second recycle stream.

9. A process according to claim 8, wherein said gaseous expanded protion of said second recycle stream is combined with said second diphasic refrigerating stream after said second part of said second and further liquid fraction, after having been expanded to said second vaporization pressure, is combined with said second diphasic refrigerating stream.

10. A process according to claim 8, wherein said second gaseous recycle stream is withdrawn from said cycle mixture in gaseous form, between the two com pression stages, under said intermediate pressure.

11. A process according to claim 8, wherein said second gaseous recycle stream is withdrawn at said higher pressure, from said residual cycle mixture in gaseous form, during the progressive and fractional condensation thereof according to step (g).

12. A process according to claim 4, wherein the refrigerating installation is conducted successively in a transient running zone and then in a permanent running zone, as the frigorific power to be supplied increases from a zero value up to a maximum value, the minimum quantity of frigorific power to be supplied during the permanent running conditions being equal to the maximum frigorific power to be supplied during the transient running conditions, and wherein (l) in permanent running conditions, the rate of flow of at least one permanent gaseous recycle stream which is withdrawn from said residual cycle mixture in gaseous form during the progressive and fractional condensation according to step (c) is controlled, said permanent recycle stream being expanded in gaseous form to said vaporization pressure and combined with said refrigerating stream after said stream has been combined with at least a part of said second and further liquid fraction (B), which has been expanded to said vaporization pressure, but before said stream has been combined with at least a part of said first liquid fraction (A), which has been expanded to said vaporization pressure, and (2) in transient running conditions, the rate of flow of at least one transient gaseous recycle stream which is withdrawn from said compressed cycle mixture in gaseous form, before its partial condensation according to step (b) but after said compressed cycle mixture has been cooled in gaseous form is controlled, said transient recycle stream being expanded in gaseous form to said vaporization pressure and combined with said refrigerating stream, when said refrigerating .stream is combined with at least a part of said first liquid fraction (A) which has been expanded to said vaporization pressure, said permanent gaseous recycle stream being controlled to a maximum value during said transient running conditions.

13. A process according to claim 8, wherein said first gaseous recycle stream and said second gaseous recycle stream are controlled successively.

14. A process according to claim 8, wherein said first gaseous recycle stream and said second gaseous recycle stream are controlled simultaneously.

15. A processaccording to claim 8, wherein the frigorific power supplied during permanent running conditions is decreased according to the following sequence of regulating steps:

1. controlling withdrawal of a first permanent gaseous recycle stream from said residual cycle mixture in gaseous form during the progressive and fractional condensation of step (g) from a minimum value including a zero value up to a maximum value, said first permanent recycle stream being expanded in gaseous form to said second vaporization pressure and combined with said second diphasic refrigerating stream before said second diphasic refrigerating stream is substantially reheated to ambient temperature,

2. controlling withdrawal of a second gaseous permanent recycle stream from said residual cycle mixture in gaseous form during the progressive and fractional condensation of step (g) from another minimum value including a zero value up to another maximum value, said second permanent recycle stream being expanded in gaseous form to said first vaporization pressure and combined with said first diphasic refrigerating stream after said first diphasic refrigerating stream has been combined with said expanded first part of said second liquid fraction (B),

and wherein the frigorific power supplied during transient running conditions is decreased by simultaneously controlling from an initial value including a zero value up to a final value (1) a first gaseous transient recycle stream withdrawn from said compressed cycle mixture in gaseous form, after said compressed gaseous cycle mixture has been cooled but before said compressed cycle mixture is partially condensed according to step (b), then successively expanded in gaseous form to said first vaporization pressure and combined with said first diphasic refrigerating stream when said first diphasic refrigerating stream is combined with at least a part of said first liquid fraction (A) which has been expanded to said first vaporization pressure, and (2) second gaseous transient recycle stream withdrawn from said cycle mixture in gaseous form under said intermediate pressure, then successively expanded in gaseous form to said second vaporization pressure and combined with said second diphasic refrigerating stream before said second diphasic refrigerating stream is substantially reheated up to ambient temperature.

16. A process according to claim 8, wherein when the frigorific power supplied increases from an initial value including a zero value up to a maximum value, there are successively reduced from an initial value to a minimum value including a zero value, (1) a first gaseous recycle stream withdrawn from said compressed cycle mixture in gaseous form, after said mixture has been cooled in gaseous form but before said compressed cycle mixture is partially condensed according to step (b), then successively expanded in gaseous form to said first vaporization pressure and combined with said first diphasic refrigerating stream, when said first diphasic refrigerating stream is combined with at least a part of said first liquid fraction (A) which has been expanded to said first vaporization pressure, and (2) a second gaseous recycle stream withdrawn from the cycle mixture under said intermediate pressure, then successively expanded to said second vaporization pressure and combined with said second diphasic refrigerating stream before said second diphasic refrigerating stream is substantially reheated up to ambient temperature.

PAGE ONE OF TWO PAGES UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 898,857 DATED August 12, 1975 lN\/ ENTOR(S) 1 JEAN BOURGUET et al It is certified that errer appears in the above-identrtied patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, line 8 thereof: replace "evaporation" with vaporisation-.

Column 1, line 43: rewrite "passages" as passage---.

Column 3, line 43: rewrite "supplied" as -supplies---.

Column 4, line 39: replace "when" with ---then-.

Column 6, line 53: replace "viii" with --vii-.

Column 7, line 38: replace "two-presence" with -twopressure.

Column 8, line 35: rewrite "communicated" as communicates.

r Column 9, line 10: rewrite "presence" as -pressure.

Column 10, line 42: before "The", delete the comma and replace with a period Column 11, line 45: replace "throughout" with throughput.

PAGE TWO OF TWO PAGES UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. I 3,898, 857

DATED August 12 1975 iN\/ ENTOR(S) Z JEAN BOURGUET et al It is certified that error appears in the ab0ve-identified patent and that said Letters Patent are hereby corrected as shown below;

Column 12, line 62: replace "frm" with from Column 12, line 65: replace "33" with --38.

Column 13, line 1: replace "explained" with --expanded.

Column 14, last line: delete the period and replace with a comma Column 15, line 19: delete the period and replace with a coma Signed and Scaled this Thirty-first Day Of January I978 [SEAL] A ttest:

RUTH C. MASON LUTRELLE F. PARKER Attesting Officer Acting Commissioner of Patents and Trademarks 

1. In a process for regulating the frigorific power delivered by a refrigeration installation operating an incorporated cascade cycle wherein a. at least one cycle mixture is compressed in gaseous form from a lower pressure to a higher pressure, in at least one compression stage, b. the resulting compressed cycle mixture is partially condensed, by heat exchange with an external refrigerant, and a first liquid fraction (A) is separated therefrom. c. progressive condensation of at least the residual gaseous cycle mixture is affected at said higher pressure by heat exchange with, and while flowing counter-currently with respect to, a diphasic refrigerating stream comprised of at least a part of said cycle mixture, which stream is undergoing vaporization at a vaporization pressure at least equal to said lower pressure, whereby at least a last liquid fraction is obtained from the residual cycle mixture, d. at least a part of each of the liquid fractions of said cycle mixture is expanded from said higher pressure to said vaporization pressure, and the expanded liquids thus obtained are combined into said refrigerating stream, and e. at least said vaporized refrigerating stream formed in (d) and constituting at least a part of said cycle mixture is recompressed in gaseous form to said higher pressure. the improvement which comprises i. withdrawing at least a first gaseous recycle stream from said cycle mixture in gaseous form, under a pressure at most equal to said higher pressure and above said lower pressure, ii. expanding to said vaporization pressure at least a gaseous portion of said withdrawn gaseous stream, iii. combining said gaseous expanded portion with said refrigerating stream, before or when said at least a part of liquid fraction (A), after having been expanded to said vaporization pressure, is combined with said refrigerating stream, iv. recompressing in gaseous form said combined portion formed in (iii) together with said refrigerating stream, when the latter has been vaporized, and v. controlling the flow-rate of said expanded gaseous portion of said first recycle stream.
 2. A process according to claim 1, wherein said expanded gaseous portion of said first recycle stream is combined with said reFrigerating stream when said at least a part of liquid fraction (A), after having been expanded to said vaporization pressure, is combined with said refrigerating stream.
 2. controlling withdrawal of a second gaseous permanent recycle stream from said residual cycle mixture in gaseous form during the progressive and fractional condensation of step (g) from another minimum value including a zero value up to another maximum value, said second permanent recycle stream being expanded in gaseous form to said first vaporization pressure and combined with said first diphasic refrigerating stream after said first diphasic refrigerating stream has been combined with said expanded first part of said second liquid fraction (B), and wherein the frigorific power supplied during transient running conditions is decreased by simultaneously controlling from an initial value including a zero value up to a final value (1) a first gaseous transient recycle stream withdrawn from said compressed cycle mixture in gaseous form, after said compressed gaseous cycle mixture has been cooled but before said compressed cycle mixture is partially condensed according to step (b), then successively expanded in gaseous form to said first vaporization pressure and combined with said first diphasic refrigerating stream when said first diphasic refrigerating stream is combined with at least a part of said first liquid fraction (A) which has been expanded to said first vaporization pressure, and (2) second gaseous transient recycle stream withdrawn from said cycle mixture in gaseous form under said intermediate pressure, then successively expanded in gaseous form to said second vaporization pressure and combined with said second diphasic refrigerating stream before said second diphasic refrigerating stream is substantially reheated up to ambient temperature.
 3. A process according to claim 1, wherein said expanded gaseous portion of said first recycle stream is combined with said refrigerating stream before said at least a part of liquid fraction (A), after having been expanded to said vaporization pressure, is combined with said refrigerating stream.
 4. A process according to claim 3, wherein by a progressive and fractional condensation according to step (c), there is obtained and separated from the residual cycle mixture at least a second and further liquid fraction, intermediate said first and last liquid fractions, and wherein said expanded gaseous portion of said first recycle stream is combined with said refrigerating stream, after at least a part of said second liquid fraction, which has been previously expanded to said vaporization pressure, is combined with said refrigerating stream.
 5. A process according to claim 1, wherein said gaseous cycle mixture is cooled after being compressed according to step (a) and before being subjected to partial condensation according to step (b), and wherein said first gaseous recycle stream is drawn off at said higher pressure from said compressed cycle mixture in gaseous form before partial condensation thereof but after said compressed gaseous cycle mixture has been cooled.
 6. A process according to claim 1, wherein said first gaseous recycle stream is drawn off at said high pressure from the residual cycle mixture in gaseous form, during the progressive condensation thereof according to step (c).
 7. A process according to claim 6, wherein by a progressive and fractional condensation according to step (c), there are obtained and separated from said residual cycle mixture second and third further liquid fractions, intermediate said first and last liquid fractions, and wherein said gaseous recycle stream is withdrawn from said residual cycle mixture in gaseous form, after the separation from said residual recycle stream of the second and third fractions.
 8. A process according to claim 1, wherein said incorporated cascade cycle includes the steps: f. compressing said cycle mixture is gaseous form according to step (a) in two compression stages separated by a conduit at a pressure intermediate said higher pressure and said lower pressure, g. progressively and fractionally condensing according to step (c), by heat exchange with a first diphasic refrigerating stream, constituting a first part of said cycle mixture, which first diphasic refrigerating stream is undergoing vaporization under a first vaporization pressure substantially equal to said intermediate pressure, and obtaining and separating from said residual cycle mixture at least a second and further liquid fraction (B), intermediate said first and last liquid fractions, h. extracting the heat content of an external fluid by heat exchange of said external refrigerant with a second diphasic refrigerating stream, constituting a second and remaining part of said cycle mixture, which second diphasic refrigerating stream is undergoing vaporization under a second vaporization pressure substantially equal to said lower pressure, j. expanding to said first and second vaporization pressures, respectively, a first part and a second and remaining part of at least said second liquid fraction (B) of said cycle mixture, obtained during the progressive and fractional condensation according to step (g); and combining the expanded first liquid part and the expanded second liquid part thus obtained with said first diphasic refrigerating stream and with said second diphasic refrigerating stream, respectively, k. recompressing in gaseous form said vaporized second diphasic refrigerating stream from said lower pressure to said intermediate pressure, and then recompressing the resulting recompressed stream in gaseous form from said intErmediate pressure to said higher pressure, together with said vaporized first diphasic refrigerating stream, and wherein the frigorific power delivered by the refrigeration installation is regulated by:
 9. A process according to claim 8, wherein said gaseous expanded protion of said second recycle stream is combined with said second diphasic refrigerating stream after said second part of said second and further liquid fraction, after having been expanded to said second vaporization pressure, is combined with said second diphasic refrigerating stream.
 10. A process according to claim 8, wherein said second gaseous recycle stream is withdrawn from said cycle mixture in gaseous form, between the two compression stages, under said intermediate pressure.
 11. A process according to claim 8, wherein said second gaseous recycle stream is withdrawn at said higher pressure, from said residual cycle mixture in gaseous form, during the progressive and fractional condensation thereof according to step (g).
 12. A process according to claim 4, wherein the refrigerating installation is conducted successively in a transient running zone and then in a permanent running zone, as the frigorific power to be supplied increases from a zero value up to a maximum value, the minimum quantity of frigorific power to be supplied during the permanent running conditions being equal to the maximum frigorific power to be supplied during the transient running conditions, and wherein (1) in permanent running conditions, the rate of flow of at least one permanent gaseous recycle stream which is withdrawn from said residual cycle mixture in gaseous form during the progressive and fractional condensation according to step (c) is controlled, said permanent recycle stream being expanded in gaseous form to said vaporization pressure and combined with said refrigerating stream after said stream has been combined with at least a part of said second and further liquid fraction (B), which has been expanded to said vaporization pressure, but before said stream has been combined with at least a part of said first liquid fraction (A), which has been expanded to said vaporization pressure, and (2) in transient running conditions, the rate of flow of at least one transient gaseous recycle stream which is withdrawn from said compressed cycle mixture in gaseous form, before its partial condensation according to step (b) but after said compressed cycle mixture has been cooled in gaseous form is controlled, said transient recycle stream being expanded in gaseous form to said vaporization pressure and combined with said refrigerating stream, when said refrigerating stream is combined with at least a part of said first liquid fraction (A) which has been expanded to said vaporization pressure, said permanent gaseous recycle stream being controlled to a maximum value during said transient running conditions.
 13. A process according to claim 8, wherein said first gaseous recycle stream and said second gaseous recycle stream are controlled successively.
 14. A process according to claim 8, wherein said first gaseous recycle stream and said second gaseous recycle stream are controlled simultaneously.
 15. A process according to claim 8, wherein the frigorific power supplied during permanent running conditions is decreased according to the following sequence of regulating steps:
 16. A process according to claim 8, wherein when the frigorific power supplied increases from an initial value including a zero value up to a maximum value, there are successively reduced from an initial value to a minimum value including a zero value, (1) a first gaseous recycle stream withdrawn from said compressed cycle mixture in gaseous form, after said mixture has been cooled in gaseous form but before said compressed cycle mixture is partially condensed according to step (b), then successively expanded in gaseous form to said first vaporization pressure and combined with said first diphasic refrigerating stream, when said first diphasic refrigerating stream is combined with at least a part of said first liquid fraction (A) which has been expanded to said first vaporization pressure, and (2) a second gaseous recycle stream withdrawn from the cycle mixture under said intermediate pressure, then successively expanded to said second vaporization pressure and combined with said second diphasic refrigerating stream before said second diphasic refrigerating stream is substantially reheated up to ambient temperature. 